ITI/NCVT Sheet Metal Worker (AITT Theory) cheat sheet

Guillotine shear safety: never place hands within 100 mm of the blade; use a push stick and keep the fixed guard adjusted to within 6 mm of the workpiece.

Hand tools: chisels and punches must be mushroom-free; dress the head at 60-70 degrees and discard if the crack extends into the body.

Fire classes for sheet metal shops: Class A (paper, wood), Class B (oil, grease), Class C (electrical), Class D (magnesium, aluminium powder). Water is forbidden on B, C and D.

Arc welding eye protection: shade 10-13 for SMAW above 100 A; shade 3-4 for gas cutting goggles; shade 5-8 for brazing and light gas welding.

Lifting rule: keep the load close to the body, back straight, knees bent; two-person lift above 25 kg per person is the usual shop limit.

Compressed air for cleaning must not exceed 2 bar (30 psi) and must carry a chip guard; never clean skin or clothing with air.

Hearing: 85 dB(A) for an 8-hour day is the action level; noise doubles with every 3 dB increase, so 88 dB halves the permitted exposure time.

Solvent and paint storage: flash point below 23 degrees C means highly flammable; store in a closed metal cabinet, bonded and earthed, away from grinding sparks.

First-angle projection (ISO/IS 696): the view placed below the front view is the top view; the view placed on the right of the front view is the left-side view.

Third-angle projection (ASME): the top view sits above the front view and the right-side view sits to the right; the symbol shows a truncated cone with the small end toward the observer.

Line conventions: continuous thick for visible outlines, short-dashed for hidden edges, chain thin for centre lines and pitch circles, continuous thin for dimension and projection lines.

Scale: representative fraction = drawing length / actual length; scale 1:5 means 100 mm on paper equals 500 mm actual.

Dimensioning rules: dimensions are given in true size regardless of scale; never dimension a feature twice; avoid dimensioning to hidden lines.

Section views: hatching lines at 45 degrees, spacing 2-4 mm; adjacent parts are hatched in opposite directions; shafts and bolts are not sectioned longitudinally.

Tolerance notation: 50 H7/g6 means hole basis with hole tolerance H7 and shaft tolerance g6; clearance exists when the shaft upper limit is below the hole lower limit.

Geometric symbols: flatness is a parallelogram, perpendicularity is an inverted T, concentricity is two concentric circles, and surface roughness Ra is written with the tick symbol.

Developed (flat pattern) views are drawn as continuous thin chain lines or marked 'FLAT PATTERN' and are never dimensioned as if they were formed parts.

Bend lines on a flat pattern are shown as chain lines; the bend allowance zone is bounded by the bend tangent lines.

Bend direction is read from the note 'BEND UP' or 'BEND DOWN' relative to the visible face; a dashed bend line indicates the bend is away from the viewer.

Sheet metal drawings usually specify material, thickness (t), inside bend radius (R) and bend angle; the neutral axis is taken at 0.33 t to 0.5 t from the inside face depending on the R/t ratio.

Hems, joggles and offsets are shown with a detail view at 2:1 or larger because the feature is smaller than the sheet thickness scale.

Sheet metal drawing callouts: '3 x 1200 x 2400 MS' means 3 mm thick mild steel sheet, 1200 mm wide, 2400 mm long.

Rivet and fastener symbols: a solid circle with a cross indicates a rivet on the far side; a filled circle indicates a rivet on the near side.

Sketching proportion: use a soft pencil (2B), keep construction lines light, and mark the datum edge first so all dimensions originate from one reference.

Mild steel sheet: density 7850 kg/m3, tensile strength 350-450 N/mm2, easily welded and formed; galvanised sheet must not be welded without removing the zinc coating first (zinc fume hazard).

Aluminium: density about 2700 kg/m3; 1050 and 1100 grades are soft and highly formable; 2024 and 7075 are strong but crack when bent sharply and need annealing.

Copper: density 8900 kg/m3, excellent ductility; anneal at 400-650 degrees C and quench in water; work-hardens rapidly and must be re-annealed between forming stages.

Stainless steel 304: work-hardens strongly, low thermal conductivity causes heat build-up; use sharp tools with small clearance and slow cutting speeds.

Tinplate: mild steel coated with tin; the coating is measured in g/m2 per side; solder joints must use non-corrosive resin flux.

Snips: straight (aviation) snips for straight cuts, curved snips for circles; the colour code is generally left-cut green, right-cut red, straight yellow.

Stakes: a hatchet stake for sharp bends, a blowhorn stake for tapering work, a beakhorn stake for curves, a creasing stake for seams.

Hammers: a ball-peen hammer for riveting and chipping, a setting hammer for lock seams, a stretching hammer with a crowned face for planishing.

Steel rule is read to 0.5 mm; vernier calliper to 0.02 mm; micrometer to 0.01 mm; a try square checks 90 degrees and a bevel protractor reads to 1 minute of arc.

Dividers transfer and step off equal distances; trammels are used for radii above about 150 mm; hermaphrodite callipers transfer internal and external sizes.

Scribing: use a scriber on bare metal, a fine-pointed pencil on coated or soft metal; a centre punch mark is made before drilling to prevent wander.

Allowance rules: add bend allowance at each bend, add 1-2 mm per edge for trimming, and never mark on a surface that will be visible after forming.

Circumference development of a cylinder: L = pi x D, where D is the mean diameter (outside diameter minus t for outside dimensions).

True length is found by rotating an inclined line to parallel the reference plane; the auxiliary view gives the true shape of an inclined face.

Layout datum: choose one straight edge as the reference, mark all dimensions from it, and check the diagonal of a rectangle to confirm squareness.

Centre of a circle by chords: draw two chords, bisect each with perpendicular bisectors; the intersection is the centre.

Guillotine shear clearance is about 5-8% of sheet thickness for mild steel and 3-5% for aluminium; excessive clearance causes burrs and roll-over.

Shearing force F = L x t x shear strength; for mild steel take shear strength as about 0.8 of tensile strength (roughly 300 N/mm2).

Blade rake angle on a guillotine is 1-3 degrees; the rake reduces peak force but introduces a slight bow in narrow strips.

Hand forming: stretching thins the metal and is done with a cross-peen hammer on a sandbag; shrinking thickens it and requires a shrinking hammer or a tuck fork.

Bending over a stake: keep the work flat against the stake face, strike with a mallet for soft metals and a hammer for steel, and move the work progressively along the bend line.

Cutting a circle by hand: drill a pilot hole, cut with curved snips clockwise, keep the waste side to the left, and file the edge after cutting.

Nibbling and filing: file with a smooth-cut file for finishing; draw-file with chalk to prevent pinning; keep the file moving in one direction only.

Deburring is mandatory after shearing; a burr left on a cut edge is a cut hazard and a stress raiser in a formed part.

Wire edge (beaded edge): turn 2-3 mm of the edge, insert a wire of the required diameter, and hammer-roll the metal over the wire to lock it.

Single seam: turn each edge in opposite directions by 6-10 mm, interlock, and flatten; a double seam folds the interlocked seam once more for a gas-tight joint.

Grooved seam: interlock the turned edges over a mandrel and indent with a grooving tool at 5-10 mm pitch to lock the joint.

Lock seam allowance: for a single seam, add 2.5 t to each edge (t = thickness); for a double seam, add about 4 t in total.

Pittsburgh lock: a proprietary interlocking seam used for ductwork; the corner is folded and hammered flat to make an air-tight joint.

Riveted joint types: lap joint, butt joint with single or double strap; pitch is the centre distance between adjacent rivets and must not exceed 3 d for tight joints.

Rivet diameter rule: d = 1.2 x square root of t for sheet metal work, where t is the sheet thickness in mm; the hole is drilled 0.1-0.2 mm larger than the rivet.

Self-tapping screws and blind rivets are used where only one side is accessible; blind rivet grip range must cover the total material thickness.

Bend allowance BA = pi x (R + K x t) x A / 180, where A is the bend angle in degrees, R is the inside radius, t the thickness and K the neutral-axis factor (0.33 for R/t < 1, 0.5 for R/t > 3).

Bend deduction = 2 x outside setback minus bend allowance; outside setback OSSB = (R + t) x tan(A/2).

Minimum bend radius: about 1 t for soft mild steel, 0.5 t for annealed aluminium, 1-2 t for half-hard aluminium, 2-3 t for 304 stainless steel.

Springback increases with the R/t ratio and with yield strength; compensate by over-bending 2-8 degrees or by coining the bend.

Roll forming: the roll gap equals the sheet thickness; the roll radius must be at least 3 t to avoid cracking; slip rolls are used for cylinders and cones.

Press brake: V-die opening should be 6-12 t; the required force per metre is approximately (1.42 x tensile strength x t squared) / V for air bending.

Folding: a folding bar and a folder are used for straight edges; the minimum flange length is about 4 t plus the inside radius to prevent the punch from slipping off.

Joggle (offset) is formed to allow two sheets to overlap flush; the joggle depth equals the sheet thickness plus 0.2 mm clearance.

Parallel line development applies to prisms and cylinders: divide the girth into equal parts, project each division to the elevation, and lay out the stretch-out equal to the girth.

Radial line development applies to cones and pyramids: find the true slant height, draw an arc of radius equal to the slant height, and set the arc length equal to the base circumference.

Triangulation applies to oblique and transition pieces: divide the surface into triangles, find the true length of each side by auxiliary views, and build the pattern triangle by triangle.

Cylinder development: stretch-out = pi x D; for a truncated cylinder, project each element length from the elevation to the stretch-out.

Cone development: sector angle theta = 360 x R / L, where R is the base radius and L is the slant height; slant height L = square root of (h squared + R squared).

Square-to-round transition: divide the round into 12 equal parts, triangulate between the four corners of the square and the 12 points, and add a seam allowance of 10-15 mm.

Frustum of a cone: develop the full cone, then subtract the small cone; the pattern is an annular sector whose widths equal the true slant length of the frustum.

Pattern allowances: add seam allowance (10-15 mm), notch allowance for corners, and wire-edge allowance of 2.5 d where a wire is to be inserted.

SMAW polarity: DCEN (straight polarity) gives deeper penetration and is used for thicker sheet; DCEP (reverse polarity) gives shallower penetration and better cleaning action.

Sheet metal arc welding uses low current (60-110 A for 1.5-2 mm sheet) and a short arc; excessive current causes burn-through on thin sheet.

GTAW (TIG) with DCEN is preferred for thin stainless steel and aluminium (aluminium needs AC to break the oxide film); use a thoriated tungsten electrode ground to a point.

Oxy-acetylene flame types: neutral (1:1 oxygen to acetylene) for steel, oxidising for brass, carburising for aluminium and for case-hardening.

Brazing filler melts above 450 degrees C but below the parent metal melting point; use a flux and a capillary gap of 0.05-0.2 mm; silver solder (BCuP, BAg) is common for copper and brass.

Soldering: soft solder melts below 450 degrees C (60/40 tin-lead melts at 183 degrees C); use zinc chloride flux for steel, resin flux for electrical work, and never use acid flux on electronics.

Riveting: the rivet shank length must equal the total plate thickness plus 1.5 d for a snap head; the hole is drilled, not punched, for tight joints to avoid cracking.

Rivet head types: snap (universal) head for general work, countersunk head for flush surfaces, pan head for light sheet; the tail is formed with a rivet set and a snap.

Guillotine shear: rated by maximum thickness and length; the hold-down must clamp the sheet before the blade descends; the back gauge sets the cut width.

Press brake: tonnage per metre is the limiting factor; the ram stroke, die opening and bend length determine the maximum bendable thickness.

Slip roll (pyramid roll): three rolls, the top roll is adjustable for thickness; the minimum rolling diameter is approximately the top roll diameter plus 3 t.

Power shears and nibblers: the nibbler cuts at about 1-2 m/min and needs a starting hole of at least the punch diameter; the punch and die clearance is 5-10% of thickness.

Hand shear (aviation snip) versus powered: powered tools require the guard in place, the workpiece clamped, and no gloves near rotating parts.

Drilling: cutting speed for mild steel with HSS drill is 25-30 m/min; feed 0.05-0.15 mm per revolution; use coolant for deep holes and for stainless steel.

Grinding: the tool rest must be set within 2 mm of the wheel and the tongue guard within 2 mm; the wheel must be ring-tested before mounting.

Spot welding: current 5-10 kA, electrode force 2-4 kN, weld time 0.1-0.5 s; the electrode tip diameter sets the nugget diameter, typically 4-5 times the square root of t.

Mass of a sheet: mass (kg) = length (m) x width (m) x thickness (mm) x density (kg/m3) / 1000; for mild steel use 7.85 kg per m2 per mm of thickness.

Blank size = developed length plus bend allowances plus seam and trimming allowances; add 5-10% wastage for irregular nesting.

Costing formula: total cost = material cost + labour cost + machine hour rate x hours + overhead percentage + profit margin.

Machine hour rate = (depreciation + power + maintenance + space cost) / annual productive hours.

Cutting speed and time: time = length of cut / feed rate; for shearing, add handling time of about 0.5-1 min per cut.

Nesting efficiency = total blank area / sheet area x 100%; a good nesting for rectangular blanks is 80-90%.

Welding cost estimation: consumable mass = deposit mass x (1 + spatter loss); deposition efficiency is about 60-70% for SMAW and 90-95% for GMAW.

Labour costing: direct labour = time taken x hourly rate; add idle time and setup time amortised over the batch quantity.

Communication: the 7 Cs are clear, concise, concrete, correct, coherent, complete and courteous; a technical report must state the observation, the measurement and the conclusion.

Interview and resume: a resume lists qualification, trade skills, tools handled, safety training and references; a cover letter is one page and specific to the employer.

Teamwork and 5S: sort, set in order, shine, standardise and sustain; 5S is the shop-floor housekeeping method used before quality audits.

Kaizen and PDCA: Plan-Do-Check-Act is the improvement cycle; Kaizen means continuous small improvement by the work team.

Computer basics: a byte is 8 bits; 1 KB = 1024 bytes; 1 MB = 1024 KB; ROM is non-volatile and holds the BIOS, RAM is volatile working memory.

Spreadsheet use for estimation: a formula begins with '='; relative references change on copy (A1) while absolute references do not ($A$1); SUM, AVERAGE and ROUND are the common functions.

CAD and CNC basics: G-code G00 is rapid positioning, G01 is linear interpolation, G02 and G03 are clockwise and counter-clockwise arcs; M03 starts the spindle clockwise.

Email and digital safety: use a strong password, never share OTPs, verify attachments before opening, and keep a backup of trade documents and drawings.